1. Field of the Invention
The present invention relates generally to insertion type gas flow meters and, more particularly, to a high temperature insertion type flow element with a mounting extension.
2. Description of the Related Art
The instrumentation and process control industry has recognized the use of the Pitot tube as a reliable device for measuring the volumetric flow of both liquids and gasses for many years. The Pitot tube operates based upon the principal that when a fixed probe is inserted into piping or duct work containing a moving fluid, the total pressure sensed by the probe is the sum of the static pressure exerted by the fluid, whether in motion or at rest, and the dynamic pressure equivalent to the kinetic energy of the fluid in motion. Conventional Pitot tube arrangements provide measurement of both the static and total pressure of the flowing fluid, the difference between which is the dynamic pressure. This differential pressure, i.e. the dynamic pressure, is directly related to and can be used to calculate the linear flow rate within the piping or duct work. The volumetric flow rate of the fluid is determined by multiplying the linear flow rate by the cross-sectional area of the conduit.
The Pitot tube is particularly useful in measuring gas flows in pipes or ducts with a large cross-sectional area because a Pitot tube causes negligible pressure loss within the conduit. In application, it is well known that flow rates, and thus dynamic pressures, within a conduit are not uniform. Affected by variables such as the Reynolds number of the particular gas and turbulence caused by wall surface roughness, dampers, elbows and other fittings, the flow rate/dynamic pressure is generally higher toward the center of the conduit and lower towards the outer extremes. This phenomenon is described in terms of a velocity profile, wherein a vector representation of the linear velocities at various points within the conduit defines a characteristic profile curve. The dynamic nature of the velocity profile precludes accurate measurement with a single Pitot tube. Rather, an accurate measurement of the flow within the conduit is obtained by placing Pitot tubes at various positions on a cross-sectional plane, sampling the dynamic pressure at various points across the velocity profile, averaging them, and using the result to calculate a volumetric flow rate.
A popular type of Pitot tube arrangement is that of the insertion type flow elements. Typical insertion type flow elements consist of a dynamic pressure sensing Pitot tube and a static pressure sensing Pitot tube connected to an element head having connections that allow for connecting the individual tubes to instrumentation devices. Depending upon the size of the duct, any number of elements can be inserted, traversing the interior cross-section of the duct at varying locations so as to account for the flow profile within the duct. The individual dynamic and static pressures are then joined at a common header or manifold so that an overall differential pressure can be determined. Use of the insertion type flow element is advantageous in that the element is suited for insertion through a duct wall and thus requires minimal installation efforts. Depending upon variables such as the size of the duct, temperature and velocity of the gasses passing therethrough, the insertion type flow element is fit with outboard support wherein it is anchored to the duct wall both at the element head end and at the opposite end of the Pitot tubes. In high temperature applications, however, a vast majority of insertion type flow element installations are of the type having an outboard support. While the insertion type flow element provides accurate measurements and ease of installation, its use does give way to some persistent problems that have, until now, gone unsolved.
One particular problem can occur in airflow applications in which particulate concentrations are heavy. In such applications Pitot tubes can become clogged, creating inaccuracies in flow measurement wherein the element senses an erroneous gas flow. In these applications, there is a need to frequently clean clogged tubes in order to maintain an accurate gas flow measurement. Typically, the element must be taken out of service for cleaning, which is labor intensive, time consuming and, thus, extremely undesirable. An adequate solution for this problem has yet to be addressed in the industry.
Another particular problem occurs in applications where the flow rate of hot gasses are measured, wherein thermal expansion causes a differential expansion between the Pitot tubes and the duct wall. When this occurs, the expansion of the Pitot tubes in the longitudinal direction is greater than that of the cross-sectional expansion of the duct wall. Being that the element is typically fit with outboard support, as a result, the Pitot tubes either fail themselves or the welded bead holding them in place fails. The present invention is directed toward a high temperature mounting extension for use in conjunction with an insertion type Pitot tube flow element having an outboard support that incorporates high temperature packing that allows for thermal differential expansion that remedies the above mentioned problems associated with conventional installation practices.
A search of the prior art produced the following inventions related to Pitot tube gas flow measuring devices:
U.S. Pat. No. 1,250,238, issued in the name of Spitzglass; PA1 U.S. Pat. No. 3,685,355, issued in the name of DeBaun; PA1 U.S. Pat. No. 4,297,900, issued in the name of Brandt, Jr.; PA1 U.S. Pat. No. 4,344,330, issued in the name of Renken et al.; PA1 U.S. Pat. No. 4,602,514, issued in the name of Kurrle et al.; PA1 U.S. Pat. No. 4,750,370, issued in the name of Ossyra; PA1 U.S. Pat. No. 5,481,925, issued in the name of Woodbury; and PA1 U.S. Pat. No. 5,483,839, issued in the name of Meunier.
While all of these patents describe devices incorporating the use of a Pitot tube device to determine the volumetric flow rate of gasses in a conduit or the like, none of them address the specific problems associated with tube cleaning or differential thermal expansion in an insertion type flow element.
Also, of considerable relevance is U.S. Pat. No. 5,736,651, issued in the name of Bowers, the present inventor. In this patent, disclosed is a high temperature gas flow sensing element wherein a frame structure supports a Pitot tube array that spans its interior cross-section. The internal dimensions of the frame are the same as that of the conduit in which the gas flow is to be measured. Inserted in line with the conduit, the gasses in the conduit flow through the element, thus producing a flow measurement. The Bowers flow sensing element incorporates the use of exterior access ports to allow for cleaning of the Pitot tubes without removing the element. Additionally, pliable, high temperature packing material is used to secure the Pitot tubes within the element while permitting them to expand and contract. While many solutions to the problems associated with high temperature flow measurement are incorporated into this invention, the disclosure does not address the specific problems encountered when dealing with an insertion type element. As such, the present invention is sufficiently novel and non-obvious so as to distinguish it from the prior art, including the present inventor's own prior art.